An optical frequency shift keying (optical FSK) is a technology which applies modulation to a frequency of light and transmits variations in the frequency as a signal. An FSK signal generally carries no information on its amplitude, so that it has a feature of being less subject to a level fluctuation or noise.
An FSK system using a digital signal has been already known (see e.g. Japanese patent application laid-open No. 11-17746). However, this technology is related to shifting the frequency of the digital signal, so that the frequency of light is not shifted.
As an example which shifts a frequency of an optical signal to be outputted, there is an optical single side-band (optical SSB) modulator (Tetsuya Kawanishi, Masayuki Izutsu, “Optical frequency shifter using optical SSB modulator”, TECHNICAL REPORT OF IEICE, OCS2002-49, PS2002-33, OFT2002-30 (2002-08). An optical FSK modulator using this optical SSB modulator is known (see T. Kawanishi and M. Izutsu, “Optical FSK modulator using an integrated lightwave circuit consisting of four optical phase modulator”, CPR 2004 G-3, Tokyo, Japan, 14-16 Jan. 2004).
FIG. 4 is a diagram showing a basic arrangement of an optical FSK communication system including such an optical FSK modulator. As shown in FIG. 4, a conventional FSK communication system is composed of an FSK demodulator (101) demodulating an optical FSK signal, an FSK modulator generating the optical FSK signal, and an optical fiber (113) connecting the optical FSK modulator and the FSK demodulator. The FSK demodulator separates the optical FSK signal into an USB signal and an LSB signal, whose respective intensities are measured by a photodetector, converted into electric signals, and a difference therebetween is obtained by a subtractor (106). At this time, a dispersive medium such as the optical fiber (113) generates a dispersion, so that frequencies of the USB signal and the LSB signal are different by two times the frequency of a modulating signal, thereby generating a time difference (delay) before the USB signal and the LSB signal reach photodetectors (104) and (105) respectively (namely, the USB signal reaches the photodetector earlier). It is to be noted that “112” in FIG. 4 denotes a light source.
FIG. 5 is a schematic diagram showing such a delay. Due to this delay, there is a problem that a difference of intensities between the USB signal and the LSB signal cannot be appropriately obtained by the subtractor.